Diesel engines use a much leaner air-to-fuel ratio compared with gasoline engines. The larger amount of air in the intake gas promotes more complete fuel combustion and better fuel efficiency, and thus lower emissions of hydrocarbons and carbon monoxide than gasoline engines. However, with the higher pressures and temperatures in the diesel engine, nitrogen oxides emissions, which include nitric oxide (NO) and nitrogen dioxide (NO2) (known collectively as NOx), tend to be higher because the high temperatures cause the oxygen and nitrogen in the intake air to combine as NOx.
NOx cause a number of concerns related to the environment, such as a source of ground-level ozone or smog, acid rain, excess aqueous nutrients, and can readily react with common organic chemicals, and even ozone, to form a wide variety of toxic products. Since the 1970's, government legislation has required increasing reductions of NOx in exhaust gas emissions.
To comply with increasingly stringent government mandates regarding NOx emissions, industry has developed several NOx reduction mechanisms. Two such mechanisms involve manipulation of engine operating characteristics and implementation of after-treatment control technologies.
In general, manipulation of engine operating characteristics to lower NOx emissions can be accomplished by lowering the intake temperature, reducing power output, retarding the injector timing, reducing the coolant temperature, and/or reducing the combustion temperature. For example, cooled exhaust gas recirculation (EGR), in which a percentage of the exhaust gases are drawn or forced back into the intake and mixed with the fresh air and fuel that enters the combustion chamber, is a well known way to lower the peak flame temperatures inside the combustion chamber. Intake air dilution reduces formation of NOx by decreasing the O2 concentration in the combustion process. To a smaller degree, the air also absorbs some heat, further cooling the process. However, use of EGR increases fuel consumption.
After-treatment control technologies that treat post combustion exhaust include selective catalytic reduction (SCR). The SCR process reduces NOx to diatomic nitrogen (N2) and water (H2O) using a catalyst and anhydrous ammonia (NH3) or aqueous NH3, or a precursor that is convertible to NH3, such as urea. Typical SCR catalysts are a honeycomb or plate ceramic carrier (e.g., titanium oxide) and oxides of base metals (e.g., vanadium and tungsten), zeolites and other precious metals.
In addition to NOx emissions, a further disadvantage of diesel engines is the production and emission of exhaust particulate matter (PM), or soot, which is produced comparatively larger amounts than that of gasoline engines. PM is a complex emission that includes elemental carbon, heavy hydrocarbons derived from the fuel, lubricating oil, and hydrated sulfuric acid derived from the fuel sulfur. Diesel PM includes small nuclei mode particles having diameters below 0.4 μm and their agglomerates of diameters up to 1 μm. PM is formed when insufficient air or low combustion temperature prohibits the complete combustion of free carbon. As such, PM is partially unburned fuel or lubrication oil, and is often seen as black smoke.
The fine particles that make up PM emissions in diesel exhaust can penetrate deep into the lungs and pose serious health risks including aggravated asthma, lung damage, and other serious health problems. PM emissions from diesel engines also contribute to haze, which restricts visibility. Due to their damaging effects, governmental agencies have imposed increasingly stringent restrictions for PM emissions.
One after-treatment device used to reduce or remove PM in diesel exhaust is a diesel particle filter (DPF). A DPF system typically includes a filter encased in a canister that is positioned in the diesel exhaust stream. The filter is designed to collect PM while allowing exhaust gases to pass through it. Types of DPFs include ceramic and silicon carbide materials, fiber wound cartridges, knitted fiber silica coils, wire mesh and sintered metals. DPFs have demonstrated reductions in PM by up to 90% or more. DPFs can also be used together with a diesel oxidation catalyst (DOC) to reduce HC, CO and soluble organic fraction (SOF) of PM in diesel exhaust.
While DPFs are very effective in removing PM from diesel exhaust gas, the volume of PM generated by a diesel engine is sufficient to fill up and plug a DPF in a relatively short time. Thus, a process cleaning or replacing the DPF must be periodically performed to allow continued engine operation. One DPF cleaning process known as regeneration burns off or “oxidizes” PM that has accumulated in the filter. However, because diesel exhaust temperatures often are not sufficiently high to burn accumulated PM, various ways to raise the exhaust gas temperature or to lower the oxidation temperature have been utilized.
Regeneration can be accomplished passively by adding a catalyst to the filter. For example, a base or precious metal coating applied to the filter surface can reduce the ignition temperature required for oxidizing accumulated PM. A DOC can be provided upstream of the DPF to oxidize NO to generate NO2 (requiring accurate control to maintain the mass ratio of NO/PM in engine-out exhaust gas), which in turn oxidizes the PM in the downstream DPF. Alternatively, regeneration may be accomplished actively by increasing the exhaust temperature through a variety of approaches, e.g., engine management, a fuel burner, resistive heating coils or late fuel injection. Active systems use pulses of diesel fuel late in the combustion cycle to oxidize across the catalyst thereby heating the DPF and oxidizing trapped PM. However, running the cycle too often while keeping the back pressure in the exhaust system low can result in excess fuel use.
Engine control modules (ECM's) (also known as engine control units (ECU's)), control the engine and other functions in the vehicle. ECM's can receive a variety of inputs to determine how to control the engine and other functions in the vehicle. With regard to NOx and PM reduction, the ECM can manipulate the parameters of engine operation, such as EGR and fuel injection.
ECM's can also control the operating parameters of exhaust after-treatment devices, such as a urea based SCR system, a DOC system, or a DPF system. For instance, an ECM can meter urea solution into the exhaust stream at a rate calculated from an algorithm which estimates the amount of NOx present in the exhaust stream as a function of engine operating conditions, e.g., exhaust flow, temperature and NOx concentration. As a further example, an ECM can monitor one or more sensors that measure back pressure and/or temperature, and based on pre-programmed set points, the ECM can activate a regeneration cycle.
In addition to EGR, designing electronic controls and improving fuel injector systems to deliver fuel at the best combination of injection pressure, injection timing, and spray location allow the engine to burn fuel efficiently without causing temperature spikes that increase NOx emissions. For instance, controlling the timing of the start of injection of fuel into the cylinders impacts emissions as well as fuel efficiency. Advancing the start of injection, so that fuel is injected when the piston is further away from top dead center (TDC), results in higher in-cylinder pressure and higher fuel efficiency, but also results in higher NOx emissions. On the other hand, retarding the start of injection delays combustion, but lowers NOx emissions. Due to the delayed injection, most of the fuel is combusted at lower peak temperatures, reducing NOx formation.
With EGR engines, one of the key components to emissions control is the turbocharger. Most manufacturers using EGR technology have developed versions of variable geometry turbochargers (VGT), which are designed to regulate the flow of cooled exhaust air back into the combustion chamber, depending on the engine's speed. The precise amount of exhaust gas that must be metered into the intake manifold varies with engine load. High EGR flow is generally necessary during cruising and mid-range acceleration, when combustion temperatures are typically very high. On the other hand, low EGR flow is needed during low speed and light load conditions. No EGR flow should occur during conditions when EGR could negatively impact engine operating efficiency or vehicle drivability, e.g. during engine warm up, idle, or wide open throttle.
Reducing NOx by manipulating engine operation generally reduces fuel efficiency. Emissions target for lower NOx have put a lot of emphasis on reduced engine out NOx to enable meeting the stringent tailpipe out NOx levels to be compliant. In doing so, many levers like charge flow, EGR flow, injection timing have been changed with the aim of reducing NOx, but on the flip side it has lowered BSFC.
Moreover, mere manipulation of engine operation may not sufficiently reduce the amount of NOx to mandated levels. As a result, after-treatment systems, such as those utilizing SCR, DOC and/or DPF elements as described above also need to be implemented. Fuel efficiency benefits of 3 to 10% can result from using SCR systems to reduce NOx rather than manipulating engine operation for NOx reduction, which negatively impacts fuel efficiency.
Urea-based SCR systems can be viewed according to four major subsystems: the injection subsystem that introduces urea into the exhaust stream, the urea vaporization and mixing subsystem, the exhaust pipe subsystem, and the catalyst subsystem. A diesel vehicle must carry a supply of urea solution for the SCR system, typically 32.5% urea in water by weight. The urea solution is pumped from the tank and sprayed through an atomizing nozzle into the exhaust gas stream. Complete mixing of urea with exhaust gases and uniform flow distribution are critical in achieving high NOx reductions. Urea-based SCR systems use gaseous ammonia to reduce NOx. During thermolysis, the heat of the gas breaks the urea (CO(NH2)2) down into ammonia (NH3) and hydrocyanic acid (HCNO). The ammonia and the HCNO then meet the SCR catalyst where the ammonia is absorbed and the HCNO is further decomposed through hydrolysis into ammonia. Alternatively, anhydrous NH3 or aqueous NH3 may be used as the SCR ammonia source. Regardless of the NH3 source for the SCR system, when the NH3 is absorbed, it reacts with the NOx to produce water, oxygen gas (O2), and nitrogen gas (N2).
The amount of ammonia injected into the exhaust stream is a critical operating parameter. The required ratio of ammonia to NOx is typically stoichiometric and must be maintained to assure high levels of NOx reduction. However, the SCR system can never achieve 100% NOx reduction due to imperfect mixing, etc.